The present invention relates generally to investigating earth formations traversed by a borehole. More particularly, the invention relates to methods for determining in situ the characteristics of hydrocarbons in a formation.
Prior to the disclosures of the parent applications hereof, lithological analyses of an earth formation which included mineral quantifications were not conducted by those skilled in the art solely by the logging of the earth formation. Such mineralogical analyses typically required core analysis. The disclosures of the parent applications, however, taught in broad terms that a mineralogical analysis of a formation could be obtained by logging the earth formation to find elemental quantities, and then by using the elemental information in an element-mineral transform to supply mineral quantities. The details of how to obtain the element-mineral transform were taught in U.S. Ser. No. 262,133 (parent application Ser. No. 574,481, now abandoned), as Well as in continuation-in-part applications therefrom, now issued U.S. Pat. Nos. 4,712,424, 4,722,220, which are hereby incorporated by reference herein.
Also taught in parent application Ser. No. 574,753 was that certain characteristics of the oil in the formation such as oil API gravity could also be determined via logging. In particular, it was recognized that if the total vanadium content of the formation could be determined by a logging tool, by knowing the mineralogical content of the formation and the relationship between certain minerals and vanadium, the vanadium content of the rock matrix could be determined; with the vanadium in the oil (the oil vanadium content corresponding to oil API gravity through a known equation) representing the difference between the determined total vanadium content and the vanadium in the rock matrix. In other words, using the teachings of the parent applications, a mineralogical analysis of the formation can be conducted by obtaining elemental concentrations and using an element-mineral transform. From a knowledge of the minerals of the formation, and using equations typically obtained via a multiple linear regression analysis, the quantity of a particular element due to its appearance in the minerals determined to be in the formation may be found. Then, knowing the total quantity of the particular element present in the formation as determined through elemental logging, and the quantity of the particular element in the rock matrix (minerals), the amount of the particular element in the formation fluids (e.g. oil) is easily determined.
While the parent application Ser. No. 574,753 particularly described a method for determining the vanadium content of oil in the formation, and a method for determining oil quality (API gravity) therefrom, the application did not detail that other chemical elements which are indicative of oil quality and characteristics could similarly be utilized.